The month of Cheshvan is also called "Mar Cheshvan." Mar means bitter, alluding to the fact that this month contains no holidays.
Sea also means drop (of water), alluding to the special connection of this month with the rains, since on the seventh day of Cheshvan one begins to ask for rain in the Land of Israel.
The Talmud recounts that fifteen days after the end of the Sukkot festival (7th of Cheshvan) people began to pray for rain, since by that time all the Jews who had gone to spend the holidays in Jerusalem, in the times of the Great Temple, had already returned to their homes.
Outside of Israel, the prayer for rain begins on December 5th or 6th each year, depending on whether it is a leap year. This date marks the end of the summer season (in the Northern Hemisphere).
Furthermore, the Great Flood of parashat Noach began on the 17th of Cheshvan.